[llvm] sketch idea of getConstantBuildVector (PR #180074)

[Simon Pilgrim via llvm-commits]

================
@@ -2194,6 +2194,46 @@ SDValue SelectionDAG::getMaskFromElementCount(const SDLoc &DL, EVT DataVT,
                  getConstant(0, DL, IdxVT), getElementCount(DL, IdxVT, EC));
 }
 
+SDValue SelectionDAG::getConstantBuildVector(EVT VT, const SDLoc &DL,
+                                             ArrayRef<APInt> Ops,
+                                             const APInt &UndefElts) {
+  assert(VT.isFixedLengthVector() && "Expected fixed-length vector type");
+  assert(VT.getVectorNumElements() == Ops.size() &&
+         "Mismatch between VT element count and Ops size");
+  assert(UndefElts.getBitWidth() == Ops.size() &&
+         "Mismatch between UndefElts width and Ops size");
+  unsigned NumElts = VT.getVectorNumElements();
+  EVT EltVT = VT.getVectorElementType();
+  assert(EltVT.isInteger() && "Expected integer element type");
+  // Determine the type to use for the constant elements. After type
+  // legalization, the original scalar element type may be illegal (e.g., i32
+  // on LoongArch64 where only i64 is legal). In that case, we promote to a
+  // legal type and rely on BUILD_VECTOR's implicit truncation.
+  EVT ConstantVT = EltVT;
+  if (NewNodesMustHaveLegalTypes) {
+    auto Action = TLI->getTypeAction(*getContext(), EltVT);
+    if (Action == TargetLowering::TypePromoteInteger)
+      ConstantVT = TLI->getTypeToTransformTo(*getContext(), EltVT);
+  }
+  SmallVector<SDValue, 16> BVOps(NumElts);
+  for (unsigned i = 0; i < NumElts; ++i) {
----------------
RKSimon wrote:

(style)
```suggestion
  for (unsigned I = 0; I != NumElts; ++I) {
```

https://github.com/llvm/llvm-project/pull/180074